Understanding genetics involves understanding the basic components of heredity. The basic unit of heredity is the gene, which is a segment of DNA found on a chromosome. But what are these biological entities? And how do they relate to one another in complex genetic processes?
DNA and the genetic code
DNA stands for deoxyribonucleic acid, a double-stranded helical structure found in the cells of living organisms. DNA comprises a sugar-phosphate backbone with nitrogen bases. Human DNA includes four such nucleotides: thymine (T), guanine (G), adenine (A), and guanine (G). The order of the bases in the DNA strand creates the genetic code, and the whole DNA content of an organism is its genome, which is passed from parent to offspring via reproduction.
Genes and the expression of DNA sequences
The traditional view of a gene is that it is a discrete unit of inheritance, but sequencing projects, such as the Human Genome Project, have shown this to be too simplistic. A new definition was provided by Gerstein et al. in the journal Genome Research in 2007: “A gene is a union of genomic sequences encoding a coherent set of potentially overlapping functional products”. In short, a gene is the DNA sequence that results in a functional product that plays a role in biological processes, such as a protein. A gene consists of the coding DNA as well as regulatory DNA sequences, and now it is understood to also include variations in chemical alterations to the backbone of the DNA, creating a new field called epigenetics.
Chromosomes and DNA packing
According to the Human Genome Project, humans have approximately 21,000 protein-coding genes, and each DNA molecule in a human cell would be 6 feet long if stretched out to its full length. So in order for the DNA needed to encode an organism to fit inside the nucleus of a cell, it has to be packed tightly. These packed DNA structures are chromosomes, the number of which can vary by organism. The human genome is divided into 23 pairs of chromosomes.
In order to compact the DNA, the helix is tightly coiled around proteins called histones, creating the thread-like structure of the chromosome, which is then coiled upon itself and constricted with protein structures called centromeres to create the characteristic X shapes associated with some chromosomes. However, the chromosome is not heterogenous; not all parts of a chromosome are as tightly packed as others. Sections of the DNA strand in which genes are actively being transcribed create loosely packed regions called euchromatin. More tightly packed regions are called heterochromatin. See a microscopic image of the difference from Yale Medical School. The heterochromatin was once considered to be a 'junk' region because of a lack of gene transcription, but a lot has to come to light in recent years about the importance of the entire genome, not just highly active regions.
DNA, genes and chromosomes
Chromosomes are passed from parent to offspring, defining the organism based on the products of gene expression. However, DNA isn’t always in the form of chromosomes; for example, DNA present in the mitochondria is passed from mother to offspring. In less complex organisms the DNA may even be a circular plasmid or linear DNA strand free in the cytoplasm. To put it simply, chromosomes are made of DNA, which creates the gene sequence inherited from generation to generation.
If you are interested in learning more about human genes and the DNA sequence of the human genome, Genetics Home Reference has information on each of the 23 human chromosomes and mitochondrial DNA.